Methods for heating steel workpieces include indirect heating and direct heating. The indirect heating includes, for example, a furnace heating. The direct heating includes, for example, an induction heating and a direct resistance heating. In the induction heating, eddy current is applied to a workpiece by electromagnetic induction to heat the workpiece. In the direct resistance heating, electric current is applied directly to a workpiece to heat the workpiece.
According to a first related art heating method, a plate workpiece having a heating target region whose width varies along a longitudinal direction of the workpiece is heated by a direct resistance heating. The heating target region is divided into a plurality of strip-shaped segment regions arranged side by side in the longitudinal direction of the workpiece. A pair of electrodes is provided for each segment region. Electric current is applied to each pair of electrodes so that the heating target region is heated uniformly (see, e.g., JP3587501B2).
Also according to a second related art heating method, a plate workpiece having a heating target region whose width varies along a longitudinal direction of the workpiece is heated by a direct resistance heating. The heating target region of the workpiece has a width decreasing monotonously from one end toward the other end in the longitudinal direction. A pair of electrodes is placed on the wide end portion of the heating target region of the workpiece, and one of the electrodes is moved toward the narrow end portion while applying electric current to the pair of electrodes so that the heating target region is heated uniformly (see, e.g., JP2013-114942A).
According to the first related art healing method, a configuration of a heating apparatus is complicated because multiple pairs of electrodes are required for one heating target region. On the other hand, according to the second related art healing method, a heating target region can be heated uniformly by a single pair of electrodes. Thus, the configuration of the heating apparatus can be simplified.
However, when the heating target region whose width varies along its longitudinal direction is divided into a plurality of strip-shaped segment regions such that the segment regions are arranged side by side in the width direction of the heating target region, the lengths of the segment regions between the pair of electrodes are different from one another, and resistances of the segment regions are also different from one another. Electric current flowing through a segment region having a relatively long length between the pair of electrodes, that is, having relatively large resistance, is relatively small. Thus, the amount of heat generated in the segment region is relatively small. Therefore, in the second related art heating method, the temperature distribution along the width direction of the heating target region may not be uniform.